1. Field of the Invention
This invention generally relates to a Stirling engine, and more particularly, to a temperature control system for the Stirling engine.
2. Description of the Prior Art
A conventional Stirling engine system is shown in FIG. 1, wherein a high temperature expansion space 1, formed by an expansion cylinder 9 and an expansion piston 10, and a constant temperature compression space 2, formed by a compression cylinder 11 and a compression piston 12, are connected to each other through a heater 3, a heat regenerator 4 and a heat radiator (or cooler) 5. A working medium used here may be either hydrogen gas or helium gas which is fluid-tightly sealed into each device. The heater 3 for absorbing heat is located within a combustion chamber 6 in which the working medium is heated and the heat radiator 5 contacts the controlling fluid such as water from a pump 7 thus by such heat exchanging, the high pressure working medium discharges heat generated by the compression operation. The heat taken by the water is then discharged to the exterior through a radiator 8. The force or energy generated within the expansion and compression spaces due to the expansion and compression operation of the high pressure gas therein may be taken from the reciprocating movement of the pistons in cooperation with piston rods 13 and 14 and a crank shaft 15 connected thereto.
The temperature at the walls of the tubes of the heater 3 is sensed by a sensor 16 made of a thermoelectric couple, and transmitted to a heat controller 17. The heat controller 17 compares the temperature difference between the actual temperature at the tubes of the heater 3 and a predetermined constant temperature T.sub.1 and gives a PID operation to the obtained difference. PID means proportioning, integration and differentiation. In other words, in order to eliminate the difference in temperature, an opening degree of an air flow regulating valve 18 is changed to control the air flow amount so that air for combustion may be in turn controlled. The air is supplied from a blower 20 which is operated by a belt 19 and the crank shaft 15 drive system. The controlled air is transmitted to an air-fuel rate controller 21 and the controller 21 controls also the fuel amount. Thus controlled fuel is injected into the combustion chamber 6 to be burned with the air for combustion. Numeral 23 designates gas pressure controller to control the pressure of the gas.
The predetermined temperature T may be determined by the material characteristics of the heater 3 and the gas pressure at full load condition of the engine operation. Generally, as a characteristic of the metal, there is a tendency that the more the temperature increases, the less the tension strength (S.tau.) becomes (See FIG. 2). In addition, considering that the gas pressure becomes maximum under full load conditions, the stress applied to the heater 3 becomes maximum under such circumstances. It is, therefore, desirable to determine the temperature T depending upon the strength of the heater 3 under full load conditions. Thus, such temperature T is determined based on the engine full load condition. However, it is also desirable to determine the temperature T to give as high a power output and as high an efficiency to the engine as possible.
FIG. 3 shows the relationship between the gas pressure P and the maximum allowable temperature T for the walls of the heater 3. T.sub.1 in FIG. 3 indicates a constant temperature determined only under full load conditions. As is apparent from the drawing, it is possible to set the temperature higher than T.sub.1 when the gas pressure P is in low range.
Thus, determining the temperature T as a constant value T.sub.1 is not an efficient nor a sufficient way especially when the gas temperature is low.